A so-called OFDM method has been adopted as a transmission method for a wireless LAN and digital terrestrial broadcasting. The OFDM method is highly efficient in use of frequency since the method enables a plurality of digitally modulated carriers (subcarriers) to be densely arranged while maintaining orthogonality thereof. Also, the OFDM method has a feature of preventing Inter-Symbol Interference (hereinafter, “ISI”) caused by a delay wave transmitted in a multipath channel, by setting guard intervals (hereinafter, “GI”).
The following describes modulation processing in the OFDM method that is performed on a transmission side. A transmission device generates an OFDM signal in a frequency domain in the following manner. First, the transmission device performs complex modulation on transmission data for each predetermined number of bits so as to obtain in-phase components and quadrature components. Then, the transmission device assigns each pair of the in-phase components and quadrature components to the respective subcarriers, and multiplexes the subcarriers. The transmission device generates an OFDM signal in a time domain by performing Inverse Fast Fourier Transform (hereinafter, “IFFT”) on the OFDM signal in the frequency domain. The transmission device provides GIs for the OFDM signal in the time domain, converts the frequency of the OFDM signal into Radio Frequency band (hereinafter, “RF band”), and transmits the OFDM signal at the radio frequency.
FIG. 15A shows a signal obtained by generating the OFDM signal in the time domain with use of the IFFT and providing a GI for the OFDM signal. In FIG. 15A, a useful symbol 1502 refers to the OFDM signal in the time domain that is generated by the IFFT, a GI 1501 refers to a signal provided as GI, and an OFDM symbol 1503 is a signal composed of the useful symbol 1502 and the GI 1501. A useful symbol period Tu is the duration of the OFDM signal in the time domain that is generated by the IFFT, a GI period Tg is the duration of the GI, and a symbol period Ts is the duration of one symbol of the OFDM signal that is transmitted. It is assumed here that Ts=Tg+Tu. The GI 1501 is provided by copying, to the frontward part of the OFDM signal in the time domain generated by the IFFT, the backward part of the OFDM signal having a length worth the GI period Tg in the time domain generated by the IFFT. The GI 1501 attached to the frontward part of the useful symbol 1502 in the above-described manner is also referred to as “Cyclic Prefix (CP)”.
FIG. 15B shows the OFDM signal transmitted from the transmission device. As shown in FIG. 15B, the transmission device sequentially transmits, as the OFDM signal, a plurality of OFDM symbols generated as shown in FIG. 15A.
The following describes demodulation processing in the OFDM method that is performed on a reception side. The reception device performs the demodulation processing by performing on a received signal an opposite process from that of the transmission device. The reception device generates the OFDM signal in the baseband in the time domain, from the received signal. Then, the reception device converts the OFDM signal in the time domain into the OFDM signal in the frequency domain, by performing Fast Fourier Transform (FFT) on the OFDM signal in the time domain on a symbol-by-symbol basis. The reception device plays back transmission data by demodulating, in units of subcarriers, the OFDM signal in the frequency domain.
In the FFT processing performed on the OFDM signal in the time domain, a time window (hereinafter, “FFT window”) having the duration of the useful symbol period Tu is set, and the FFT processing is performed on the OFDM signal in the time domain in the time window. At this time, it is necessary to appropriately set the time position (hereinafter, “FFT window position”) of the FFT window. Otherwise, Inter Career Interference (hereinafter, “ICI”) in the same symbol and ISI from adjacent symbols occur.
FIGS. 16A and 16B are each a schematic diagram showing an example of the FFT window set for the received OFDM signal. In the following explanation, the FFT window position is designated by the forefront (shown by black triangles in FIGS. 16A and 16B) of the FFT window. The FFT window is shown by a region surrounded by a dotted line in each of FIGS. 16A and 16B, and has a predetermined period (useful symbol period Tu) starting from the FFT window position.
As shown in FIG. 16A, when the FFT window position is set at the start of a GI period, the FFT processing is performed on a part of the OFDM signal corresponding to a period including (i) the entire GI period and (ii) a period from the start of the useful symbol to Tu—Tg. Also, as shown in FIG. 16B, when the FFT window position is set at the start of the useful symbol, the FFT processing is performed on a part of the OFDM signal corresponding to the entire useful symbol period.
The following describes how ICI and ISI occur during the demodulation process of the OFDM signal in which the FFT is performed in the above-described manner, with reference to FIGS. 17A and 17B and 18.
FIGS. 17A, 17B, and 18 each schematically show the OFDM signal received by the reception device, where the horizontal axis represents time. As shown in FIGS. 17A, 17B, and 18, the received OFDM signal includes a principal wave Sp and a delay wave Sd that is delayed by delay time τ from the principal wave due to multipath transmission. The received OFDM signal includes a plurality of consecutive symbols. The following describes the case of demodulating the Nth symbol.
FIGS. 17A and 17B each show an example where the delay time τ is shorter than or equal to the GI period Tg. In FIG. 17A, the FFT window position is set to coincide with the GI periods of the principal wave Sp and the delay wave Sd. In FIG. 17B, the FFT window position is set at the start of the GI period of the principal wave Sp.
As described above, when an incoming time difference between the principal wave Sp and the delay wave Sd is shorter than or equal to the GI period Tg, and the FFT window position is set as shown in FIG. 17A, the FFT window only includes the Nth symbol of the principal wave Sp and the Nth symbol of the delay wave Sd, and not the other symbols of the OFDM signal. As a result, ICI and ISI do not occur.
On the other hand, although an incoming time difference between the principal wave Sp and the delay wave Sd is shorter than or equal to the GI period Tg, when the FFT window position is set as shown in FIG. 17B, ICI and ISI occur. This is because an N−1th symbol signal 1701 (shown by a lattice pattern in FIG. 17B) of the delay wave Sd is included in the FFT window used for the demodulation of the Nth symbol. Also, when the FFT window position is set as shown in FIG. 17B, the duration of the component of the Nth signal of the delay wave Sd becomes shorter than the duration Tu. As a result, orthogonality is lost between a plurality of carriers that constitute the OFDM transmission signal, causing ICI to occur.
Also, when an incoming time difference τ between the principal wave Sp and the delay wave Sd is larger than the GI period Tg as shown in FIG. 18, ICI and ISI occur regardless of where the FFT window position is set. For example, when the FFT window position is set as shown in FIG. 18, an N−1th symbol signal 1801 of the delay wave Sd is included. When an incoming time difference τ between the principal wave Sp and the delay wave Sd is larger than the GI period Tg as shown in FIG. 18, the effect of ICI and ISI cannot be excluded thoroughly by changing the setting of the FFT window position. However, in order to accurately demodulate the received OFDM signal, it is very important to set the FFT window position in a manner that minimizes the occurrence of ICI and ISI.
Patent Documents 1 and 2 each disclose a technique for setting the FFT window position.
Patent Document 1 discloses a reception device having a functional structure as shown in FIG. 11. As shown in FIG. 11, in the reception device, a Fourier transform unit 1101 performs Fourier transform on a received signal, a pilot extraction unit 1103 extracts a pilot signal from the received signal on which the Fourier transform has been performed, and a first division unit 1105 divides the extracted pilot signal with use of a known signal generated by a known signal generation unit 1104 and obtains channel characteristics in the position of the pilot signal. A first delay profile estimation unit 1107 obtains a delay profile from the channel characteristics obtained by the first division unit 1105. A first timing synchronization unit 1102 determines the FFT window position based on the value of the delay profile estimated by the first delay profile estimation unit 1107, and sets the FFT window position for the Fourier transform unit 1101. At this time, the first timing synchronization unit 1102 determines, as incoming waves, the amplitude of the delay profile or components of the delay profile in which the square value of the amplitude is greater than a predetermined threshold value. Then, the first timing synchronization unit 1102 sets the FFT window position based on the incoming time of the most preceding wave among the incoming waves. The reception device of Patent Document 1 appropriately sets the FFT window position in accordance with the incoming time of the most preceding wave. In this way, when a spread in time (i.e., delay spread) of the incoming waves is smaller than or equal to the GI length, the reception device of Patent Document 1 can prevent the occurrence of ICI and ISI.
Patent Document 2 discloses another method for setting the FFT window position. As shown in FIG. 12, in a reception device in Patent Document 2, an FFT circuit 1203 acquires a signal in a useful symbol period from the OFDM signal in the baseband input from a selector 1202, with use of an FFT time window signal input from a window position control unit 1205. Then, the FFT circuit 1203 performs the FFT computation on the acquired signal. The data of a result of the FFT computation performed by the FFT circuit 1203 is equalized by an equalization circuit 1241 in a data demodulation unit 1204. The equalized data is then demodulated by a demodulation circuit 1242 and performed error correction processing by an error correction circuit 1243, and is output as demodulated data. At this time, an S/N calculation circuit 1244 calculates S/N (Signal to Noise ratio) data, with use of the output of the equalization circuit 1241. The calculated S/N data is input into the window position control unit 1205 as reception quality data indicating the reception quality of the received signal. A reception quality judgment circuit 1251 in the window position control unit 1205 compares with a predetermined reference value the value of the reception quality data that has been input. When the value of the reception quality data is greater than or equal to the reference value, it is judged that the reception quality is excellent. When the value of the reception quality data is less than the reference value, it is judged that the reception quality is poor. A time window setting circuit 1252 adjusts the set position of the FFT time window signal of the FFT circuit 1203.
Note that an input switch circuit 1253 instructs the selector 1202 to select, as input, (i) a baseband OFDM signal output from a buffer memory 1206 while the set position of the FFT time window signal is being adjusted and (ii) a baseband OFDM signal output from a quadrature detection circuit 1201 after the set position of the FFT time window signal is determined. A convergence judgment circuit 1254 gives an instruction for newly writing data into the buffer memory 1206, and also gives an instruction for reading the newly written data after the writing of the data has been completed.
Patent Document 1: Japanese Patent No. 3654646
Patent Document 2: Japanese Patent Application Publication No. 2004-336279
The method for setting the FFT window position described in Patent Document 1 is not suitable in a case shown in FIG. 18, namely a case where the delay spread of the incoming waves exceeds the GI length. This means that the electric power of a signal that causes ISI is also included in the FFT window, and the FFT window position is determined based on the incoming time of the most preceding wave, without any reference to the magnitude of the received power of each incoming wave. As a result, the method described in Patent Document 1 cannot select the FFT window position corresponding to the smallest ISI amount, depending on the reception environment.
The following explains in detail the problem that arises in the method for setting the FFT window position described in Patent Document 1, with reference to FIGS. 13A, 13B, 14A, and 14B. FIGS. 13A and 14A each show the delay profile of a three-wave multipath channel. FIG. 13B and FIG. 14B schematically show transmission symbols of the OFDM signal corresponding to FIG. 13A and transmission symbols of the OFDM signal corresponding to FIG. 14A, respectively. In each of FIGS. 13A and 14A, the horizontal axis represents the incoming time of each incoming wave, and the vertical axis represents the received power of each incoming wave. Assumed here is a case of the three-wave multipath. Each of the incoming waves is respectively referred to as an incoming wave S1 (received power: P1), an incoming wave S2 (received power: P2), and an incoming wave S3 (received power: P3), in order of incoming time. Also, based on the incoming time of the incoming wave S1, the incoming time of the incoming wave S2 is assumed to be τ2, the incoming time of the incoming wave S3 is assumed to be τ3. Also, the received power of each incoming wave is assumed to be P2>P3>P1. Here, the duration of the GI period is assumed to be Tg, where τ2<Tg, τ3>Tg, and (τ3−τ2)<Tg.
As shown in FIG. 13A, when the FFT window position setting method described in Patent Document 1 is used, the FFT window position is set at the start of the useful symbol period of the incoming wave S1 that is the most preceding wave in the delay profile. In this case, the FFT window includes an N−1th symbol signal 1301 of the incoming wave S3, as shown in FIG. 13B. Therefore, ISI occurs due to the N−1th symbol signal 1301.
Meanwhile, assume that the FFT window position is set at the start of the GI period of the incoming wave S3, as shown in FIG. 14A. In this case, by including the incoming wave S3 in the FFT window, an N+1th symbol signal 1401 of the incoming wave S1 is also included in the FFT window, as shown in FIG. 14B. Therefore, ISI occurs due to the N+1th symbol signal 1401. This example is given on the assumption that P3>P1. Therefore, in the conventional FFT window position setting method in Patent Document 1 where the FFT window is set as shown in FIG. 13B, ISI occurs in a greater amount, resulting in the reception quality being deteriorated, compared to when the FFT window is set as shown in FIG. 14B. Such deterioration is caused because the conventional FFT window position setting method in Patent Document 1 is developed in view of merely two points, namely (i) the time position of each incoming wave and (ii) whether the received power of each incoming wave or the amplitude thereof is greater than a predetermined threshold.
In the conventional FFT window position setting method in Patent Document 2, when the window position control unit 1205 in FIG. 12 searches for the optimal FFT window position, the FFT circuit 1203 needs to perform FFT computation every time the time window setting circuit 1252 sets an FFT window position. This causes a great delay in processing, and the window position control unit 1205 fails to search for the optimal FFT window position promptly. Therefore, the conventional method in Patent Document 2 is not suitable in an environment where the phase and amplitude of a fading channel, etc. change constantly.
In the conventional method in Patent Document 2, the FFT window position is set in the following manner. First, the FFT computation is performed in a certain position of a time window, so as to judge whether or not a value indicating the received quality in the position is greater than or equal to a predetermined reference value. When the value is greater than or equal to the reference value, the FFT window position is set in the position of the time window at which the FFT computation has been performed. During the processing of setting the FFT window position, signal data stored in the buffer memory is used. Therefore, if the optimal position of the time window cannot be found in several attempts, the processing of the reception device is delayed until the FFT window position is set. In addition, if it takes longer than a predetermined time period to set the FFT window position, the optimal position is re-searched after writing, into the buffer memory, an OFDM signal in a new baseband. Therefore, if the situation of exceeding the predetermined time period continues, the received quality deteriorates.
Furthermore, in Embodiment 2, Patent Document 2 discloses a reception device that includes an FFT computation circuit used for control, in addition to an FFT computation circuit used for demodulation. In this case, however, the circuit size is increased although a delay in demodulation processing time can be prevented. Also, although an additional component is provided for control, the processing time for setting the FFT window position still needs to be long enough to perform the FFT computation. Therefore, it is difficult to apply the reception device in Patent Document 2 to a channel in which amplitude and phase constantly change.
In view of the above-described problems, an object of the present invention is to provide a reception device that sets an FFT window position promptly and minimizes an ISI amount, compared to conventional techniques.